The present invention relates to methods of trimming electrical resistance elements and more particularly relates to methods of trimming electrical resistance elements for use in variable resistance devices such as potentiometers.
Frequently, resistance elements which are used in variable resistance devices, such as potentiometers, are manufactured to have a particular resistance value, within a given tolerance range, by depositing a thin layer or film of resistance material on a nonconductive substrate or support to form a resistance path having a resistance value less than a nominal, desired resistance value. Then, portions of the resistance material are removed (trimmed) from the resistance path until the desired resistance value is achieved. Various devices, such as abrasive or grinding apparatus, are available for removing the resistance material from the resistance path. However, when rapid trimming of a large number of resistance elements is required, it is preferable to use a computer controlled laser trimming system.
The scan, plunge, L and comb trims are known methods of trimming a resistance element which may be accomplished with a computer controlled laser trimming system. The scan trim comprises physically removing resistance material from the resistance element by making a series of incremental longitudinal laser cuts in the resistance material along the resistance path while measuring the total resistance of the resistance element. The longitudinal cuts are terminated when enough resistance material has been removed so that a desired total resistance value is achieved. The scan trim provides a resistance element which is precisely trimmed to a desired total resistance value. Also, the scan trim provides a resistance element which is suitable for use with conventional contact wipers which are used as part of variable resistance devices, such as potentiometers, and other similar devices. However, it is usually necessary to make a large number of longitudinal cuts in the resistance element to complete a scan trim. This is relatively time consuming and costly especially when it is necessary to trim large numbers of electrical resistance elements.
A plunge trim comprises making a single transverse cut into one side of a resistance element generally perpendicular to the resistance path formed by the element. The total resistance of the element is measured as the transverse cut is made and the cut is terminated when the measured total resistance increases to a desired value. If the initial total resistance of the element is much less than the desired value then the plunge trim may require making a long transverse cut which may extend almost completely across the resistance path.
A long transverse cut resulting from a plunge trim may be avoided by making a L-trim which combines the transverse cut of the plunge trim with a longitudinal cut to form a L-shaped cut in the resistance material. When making a L-trim the transverse cut is limited to a fixed maximum length. If the total resistance of the element is not equal to the desired value when the transverse cut is completed to its maximum length then a longitudinal cut is made to complete the trim. The longitudinal cut is terminated when the measured total resistance increases to the desired value.
The plunge trim and L-trim provide resistance elements which are precisely trimmed to desired resistance values. However, resistance elements made using the plunge trim and L-trim inherently have resistance functions with major linearity deviations. In addition, special contact wipers must be used, or modifications must be made to the resistance element, to avoid electrical and mechanical problems caused by a contact wiper riding over the cut in the resistance element.
The comb trim comprises making a series of spaced transverse cuts into one side of a resistance element to form a comb-like pattern of resistance material. The total resistance of the element is measured as each cut is made and each cut is terminated when the total measured resistance increases by a preselected, fixed amount. The number of cuts and fixed amount of resistance increase are preselected so that when all the cuts are made the total resistance of the element equals a desired resistance value. A comb trim usually requires a relatively long time to complete since a large number of transverse cuts usually are required to accurately trim an element to a desired resistance value within typical tolerance ranges. In addition, the comb trim, like the plunge trim and L-trim, requies that special measures be taken to avoid electrical and mechanical problems caused by a contact wiper riding over the cuts in the resistance element.